We are testing the hypothesis that the phase-resetting mechanism that accounts for lengthened activity period (rat's wheel running) in continuous light also determines the period of transients when illumination cycle is reversed (180 degrees phase shift) and the limits of entrainment beyond 24-hr. "days". We correlate the respective measures in a group of subjects, under each of two intensity levels. If we are able to perfect techniques for long-range monitoring of internal temperature, we will attempt to relate individual differences in this measure to previously demonstrated differences in response to high ambient temperature. In order to begin a program of circadian research in human subjects, the Principal Investigator will be spending 1975-76 at the University of Sussex, in the Experimental Psychology Laboratory.